The present invention relates to an electrophotographic color image forming apparatus in which a plurality of image exposure means and developing means are arranged along the circumferential surface of an image-forming object (a photoreceptor), the configuration of which is mainly formed to be drum-shaped, and toner images are formed and superimposed while the image-forming object is rotated by one revolution.
Concerning the method for forming a multi-color image, there have been known some methods including apparatus (A) in which photoreceptors, charging units and developing units each in number equivalent to the number of colors necessary for the multi-color image are provided, and toner images each being a mono-color formed on each photoreceptor are superimposed on an intermediate transfer object to form a color image, apparatus (B) in which one photoreceptor is rotated plural times so that charging, image exposure and developing for each color are repeated for forming a color image for each rotation, and apparatus (C) in which charging, image exposure and developing for each color are conducted in succession while one photoreceptor makes one turn for forming a color image.
However, the apparatus (A) has a drawback that the dimensions of the apparatus are increased because a plurality of photoreceptors and intermediate transfer objects are required, while the apparatus (B) has a restriction that the size of a formed image is limited to the surface area or less of the photoreceptor although the dimensions of the apparatus can be small because the required number of each of the charging means, image exposure means and photoreceptor is just one.
In the case of the apparatus (C), which makes it possible to form images at high speed, it still has a contradiction that the diameter of a photoreceptor is large and thereby the apparatus is also large due to the following two reasons; one is necessity that a plurality of charging units, image exposure means and developing units need to be arranged within a circumferential surface of the photoreceptor, and the other is necessity that the distance between the image exposure means and the developing unit needs to be long for avoiding a possibility that image quality is deteriorated by toner leaking from the developing unit to which an image exposure optical system is located close.
For the purpose of avoiding the drawback of the aforementioned contradiction in the apparatus (C), there has been suggested an apparatus in which the base of an image-forming object is formed from a transparent material, a plurality of image exposure means are housed in the image-forming object, and a light-sensitive layer formed on the external surface of the base is exposed to light reflected on an image through the base (for example, Japanese Patent Publication Open to Public Inspection No. 307307/1993).
However, the above apparatus has drawbacks including complicated structure due to the arrangement of many image exposure means provided inside the image-forming object and many charging units and developing units provided outside the image-forming object, inefficient handling due to complicated mounting and dismounting of developing units, image-forming objects and image exposure means, and difficulty of keeping positional accuracy between various units. In particular, it requires an advanced technique to provide an optical system fixed inside the image-forming object and to rotate it or to mount and dismount it.
With regard to the exposure optical system, in particular, positional relationship between various parts in the optical system and that between the optical system and the image-forming object are kept to be highly accurate. Therefore, when deformation or displacement is caused by the mounting or dismounting of the image-forming object, registration or an image forming position is changed, resulting in inability to obtain excellent color images. It is preferable to integrate developing units and others into one body to the utmost.
The above problems are solved by the present invention. The first object of the present invention is to provide a color image forming apparatus characterized in that: it is possible to replace the image-forming body without affecting the image exposure means arranged in high accuracy; the layout of units is rationalized so that the positional accuracy of each unit can be maintained high and further each unit is arranged in a well-balanced condition; and the apparatus is made compact and handy.
According to the color image forming apparatus disclosed in the aforementioned Japanese Patent Publication Open to Public Inspection No. 307307/1993, the shaft penetrates through the image-forming body and the support body composed of a heat conductor for holding 4 image exposure means is provided in the image-forming body. Therefore, it is complicated and takes time to assemble the image forming body and the image exposure means, and it is also complicated to adjust a positional relation between the charger provided on the outer circumference of the image forming body and the image exposure means.
The present invention has been accomplished to solve the above problems. The second object of the present invention is to provide a color image forming apparatus characterized in that: it is possible to form a color image, the colors of which are well-balanced, and the length of which is not less than the circumferential length of the image forming body; the image exposure means provided inside the image forming body can be easily maintained and replaced; and it is easy to adjust a positional relation between the charger and developing units provided on the outer circumference of the image-forming body and the image exposure means.
A photoreceptor drum, which is an image-forming body, is rotated in the process of image formation and subjected to image exposure. A plurality of images are simultaneously superimposed on the photoreceptor drum by a plurality of optical exposure means. Due to the fluctuation of drive and unevenness of rotation, the accuracy of superimposed dots is deteriorated, so that the dots tend to be shifted. Since the photoreceptor drum is rotated while a plurality of developing units are being pressed against a surface of the photoreceptor drum, the rotation of the photoreceptor drum is fluctuated and image formation is made in an unstable condition. Each time the photoreceptor drum is attached to and detached from the apparatus body, the positional setting of the image exposure means with respect to the optical system must be conducted, and even if the position of each unit is mechanically regulated, an optical fluctuation which can not be neglected is caused and further the optical system is damaged. Therefore, it is impossible to form a clear image.
It is the third object of the present invention to solve the above problems so that the photoreceptor drum and developing units can be stably driven and the fluctuation of the photoreceptor drum is avoided and further the photoreceptor drum is combined with a stable exposure system so as to form an image of high quality.
In general, Selfoc Lens (brand name) is used in the image exposure means, and an optical exposure system, the light source of which is an LED, is used for the optical image forming system. When the optical exposure system is included in the image forming body and the LED is lit over a long period of time, generated heat is accumulated in the image forming body. Accordingly, there is a possibility that the photoreceptor layer and toner are deteriorated by the generated heat and further the registration function is changed by the effect of thermal expansion.
In order to solve the above problems, a heat pipe is conventionally embedded in a Support for supporting the optical exposure system so that the heat generated by the LED can be emitted outside.
However, even if the heat pipe, the cost of which is high, and the structure of which is complicated, is installed, the above problems can not be solved completely. Specifically, heat in the support is mainly emitted by the action of the heat pipe, and heat in other units including the image-forming unit can not be sufficiently cooled.
The present invention has solved the above problems. It is the fourth object of the present invention to provide a color image forming apparatus characterized in that: the units inside the image-forming body are widely and effectively cooled, so that the image forming function is not deteriorated even if the apparatus is used over a long period of time and images of high quality can be successively formed.